1. Field of the Disclosure
The present disclosure relates generally to gel electrolyte materials. More particularly, the present disclosure relates to heat-resistant gel electrolyte materials and their uses, for example, in electrochromic devices such as electrochromic windows.
2. Technical Background
An electrochromic window (ECW) is a device that changes its optical properties in a reversible and persistent way (e.g., between substantially transparent and a less transparent state) upon the input of a voltage pulse. ECWs have significant potential to reduce energy use in buildings. Buildings account for roughly 40% of the world's energy use, with the resulting carbon emissions being substantially more than those from the transportation sector. In the United States, the energy lost through today's relatively inefficient window stock accounts for about 30% of building heating and cooling energy. ECWs can be a significant factor in reducing building energy use and ultimately in achieving net zero energy buildings. Dynamic windows are key to achieving this goal while preserving the view and enhancing the comfort and productivity of building occupants.
There are several possible configurations for ECWs; one practical one is shown in FIG. 1. The ECW of FIG. 1 includes a stack of seven layers. The outermost layer on each side is glass, on which a transparent conductive oxide film is coated. The conductive layer is desirably substantially transparent. On one of the conductive oxides is coated an electrochromic material; on the other, a counterelectrode material (an “ion storage film”) is coated. Between the electrochromic layer and the counterelectrode material are sandwiched an electrolyte material. The counterelectrode material is capable of electrochemically reversibly absorbing lithium ions from the electrolyte and releasing lithium ions into the electrolyte The assembled stack is sealed with an appropriate adhesive and tested for optical performance. The electrochromic material typically has mixed conductivity for both electrons and ions; if ions are introduced from electrolyte or from an adjacent ion conductor there is a corresponding charge-balancing counterflow of electrons from the transparent electron conductor. The electrons remain in the electrochromic material as long as the ions reside there and, the electrons will then evoke persistent change of the optical properties. Depending on the nature of the electrochromic material, the injected electron may increase or decrease transparency. The electrolyte can take many forms, for example, a thin film or a bulk material like a solid inorganic or organic polymeric material. The ion storage material provides cyclic stability to the ECW by maintaining ions for the next cycle. Like the electrochromic material, the counterelectrode (or “ion storage”) material typically has high ionic and electronic conductivity. The counterelectrode material may or may not have electrochromic properties. When a voltage is applied between the transparent conductors as indicated by FIG. 1, a distributed electrical field is set up and ions move into or out of the electrochromic material, causing a change in its optical properties (e.g., transparency). The charge-balancing counterflow of electrons through the external circuit then leads to a variation of electron density in the electrochromic material thereby result in modulation of their optical properties. If the electrolyte has negligible electronic conductivity, the device will exhibit open circuit memory, so that the optical properties remain stable over periods of time. The applied voltage to the ECW is desirably on the order of only a few volts, as higher voltages may lead to deterioration of the device.
In applications where relatively low weight, thickness and power consumption are desired, organic materials that can be precisely printed, sprayed, spin coated, stamped, drop-casted into predetermined patterns offer a competitive alternative to their inorganic counterparts. Many organic materials exhibit redox states with distinct electronic (UV/visible) absorption spectra. Where the switching of redox states generates new or different absorption bands in the visible region, the material is said to be electrochromic]. Color changes are commonly between a transparent (‘bleached’) state, where the chromophore substantially absorbs only in the UV region and a colored state; or between two differently-colored states. Where more than two redox states are electrochemically accessible in a given electrolyte solution, the electrochromic material may exhibit several colors and be termed poly-electrochromic.
Organic electrochromic materials are of three basic types. In Type 1 materials, the electrochromic material is soluble in both the reduced and oxidized state in a given electrolyte material. For example, 1,1-di-methyl-4,4-bipyridilium(methyl viologen) dissolves in both oxidized and reduced states. For such materials, soluble electrochemically-generated product material diffuses away and the current flow must be maintained until the whole solution becomes electrolyzed to maintain its optical properties in a given state. In Type II materials, only one of the reduced or oxidized states is soluble, for example 1,1-di-heptyl-4,4-bipyridilium(heptyl viologen). In Type III materials, such as conductive polymers, both redox states are solids; such systems are studied and used, for example, as solid thin films on substrates. Type II and III materials can have optical memory, which means that once the redox state has been switched, no further charge injection is needed to retain the new electrochromic state.
The electrolyte material is often in the form of a gel. Conventional gel electrolytes include an alkali metal salt in a polymer host (often swollen with a solvent to provide the gel properties). Electrolytes for ECWs should be highly ionically conductive but relatively electrically insulating. High ionic conductivity ensures the movement of cations and anions in the electrolyte, but if the electrolyte is electrically conductive it can short-circuit the ECW. Transmittance is another important criterion to be taken in the selection of an electrolyte material for an ECW. The electrolyte should maintain high transparency and ionic conductivity over a range of temperatures.
Accordingly, what is needed are electrolyte materials that can provide desirable heat resistance as well as desirable levels of ion conductivity, electrical insulation and transmittance.